Wave translating system



ec. 28, 1937. I .LG. KREER, JR 12,103,468 WAVE TRANSLATING SYSTEM I vFiled April 1, 1956 5/ FIG.

, INVENTOR JGKREER Patented Dec. 28, 1931 2,103,468. I wavn TRANSLATINGSYSTEM John G. Kree'r, n; Bloomfield, N. J., assignor to Bell TelephoneLaboratories Incorporated, New York, N. Y., a corporation of New YorkApplication April 1, 1936, Serial No. 71,997

' 9 Claims. (01. 119-111) This invention relates to wave translation andespecially to retroaction or feedback inwave translating systems as, forexample, in systems involving vacuum tubes or electric wave amplilyingdevices.

An object of the invention is to control trans mission properties as,forexample} modulationor transmission efflciency, in such systems.

It is also an object of the-invention to control 10 feedback in-suchsystems or to facilitate application of feedback in such systems.

A further object is to reduce singing tendency in such systems,especially amplifiers or systems involving push-pull stages that feedback a portion of the output waves'in gain-reducing phase and in amountsufilcient to reduce distortion below the distortion level withoutfeedback.

In one specific aspect, the invention is embodied ,in such an amplifierinwhich the singing tendency is a tendency to -sing around the feedbackloop c'aused by parallel operation of the push-pull vacuum tubes oramplifying elements and, in accordance with a feature of the invention,the tendency is reduced by auxiliary negative feedback without'materially loweringutheiamplifler gain.

The-auxiliary feedback can be, for'example, negative feedback around onestageor an odd number of stages of the vacuum tubes or amplifyingelements for suitably altering the propaga- 1 tion (around themainfeedback loop) of trans-.

, mission that (in passing around the main feedback'loop) passes throughthe push-pull tubes in the parallel mode of propagation, to preventsuch'transmission from resulting in objectionable tendency towardsinging aroundthe main feedbackloop.

The auxiliary feedback voltage can be derived,

for' example, from a mid-branch of a, balanced '40 circuit ofanamplifying stage, (so that ft will include only transmission passingthrough the two sides, of the balanced circuit in parallel), in order toavoid materially reducing the amplifier ,gain. v I j K 4 Initsspecific-aspect mentioned above, the inventionmafbe, fjor'exampl, atwo-stage amplifier comprising a-singleetube stage driving apushpullstage. The 'normal mode offpropagation through the push-pull stage isthe push-pull 0 mode. Transmission so propagated may be called thenormal transmission. For this normal trans mission, one of thepush-pull'tubes has its plate potential approximately opposite in phaseto the potential of the driver-tube grid. A feedback 55 connection fromthis one plate to the driver-tube grid may be provided to producenegative feedback of normal transmission. 1. e., of transmissionpropagated through the push-pull stage in the normal mode. w

The feedback loop for this negative feedback 5 may be referred to as themain feedback loop;

and the feedback around this loop of normal transmission or transmissionpropagated in the normal mode through the push-pull stage may be calledthe normal or main feedback. l

Due to unbalance of the push-pull circuit, there may be undesiredpropagation of transmission through the push-pull stage, thistransmission being prop'agatedin the parallel mode, 1. e., with the twotubes of the push-pull stage effectively 15 in parallel. "Thistransmission'may be called the 7 parallel transmission. For thisparallel transmission, the plates of the push-pull stage unfortunatelymay have their potential in phase with that of the driver-tube grid.This condition may 20 result in strong tendency towardsinging around themain feedback loop, especially since the driver stage 'maybe a high-gainstage. This. high gain may be desired, for example, in order to give'theloop gain for the normal ormain feedback 25 around the main feedbackloop (i. e., the decibel gain for. one trip of the normal transmissionaround the main feedback loop) a high value,- e. g., several times tendecibels. The large loop gain is desirable, for example, for producinglarge reduction ofdistortion. The distortion reduction required may beespecially large in case'the push-pull stage is operated as a class Bamplifier.

The singing tendency can be effectively reduced without materiallyreducing the amplifier gain, 35 for example," by producing localnegative feedback around the drivertube of the parallel transmission tothe exclusion of the normal transmission. As one example of ways inwhich this may be done, a voltage can be derived from the mid- 40 branchof the balanced input circuit of the pushpull tubes, and fed back to thedriver-tube grid in gain-reducing phase, i. e., in such phase as toreduce the derived voltage.

Other objects and aspects of the invention will be apparent from thefollowing description and i claims. g

Figs. 1, 2 and 8 are circuit diagrams of three forms of the invention.

Fig. 1 showsa two-stage push-pull amplifier for amplifying wavesreceived from circuit I and transmittingthe amplified waves to circuit2.

The waves may be, for example, speech waves or a broad band of carrierwaves transmitting a number of speech messages by multiplex carrierlinear distortion, comprising two similar vacuum tubes 3 and 3, shown byway of example, as

pentodes. The second stage may be a power stage.-

It comprises two tubes 4 and 4' which are also alike and which areshown, by way of example, as triodes.

The tubes 4 and 4' are connected to the outgoing circuit 2 throughoutput transformer Lhaving closely coupled primary windings 8 and 8.

The incoming circuit I is connected to the tubes 3 and 3' through inputtransformer I0 and bridge networks II and II. The transformer hassecondary windings I2 and I2. The winding I2 forms one diagonal ofbridge II and .the winding I2 forms one diagonal of bridge II. The fourratio arms of bridge II comprise resistances 11, km, Zcr and r,respectively, the grid v and cathode of tube 3 being connected toreceive the voltage across arm h. The four ratio arms of bridge IIlikewise comprise four resistances r1, kn, hr and r, the grid andcathode of tube 3' being connected to receive the voltage across thisresistance 11.

The normal mode of propagation of transmission through the two push-pullstages is the pushpull mode, and transmission so propagated may becalled the normal transmission.- For this normal transmission, thepotentials of the plate of tube 4 and the grid of tube 3 areapproximately opposite in phase, and the potentials of the plateof tube4' and the grid of tube 3' are approximately opposite in phase.

- A feedback path comprising two feedback connections I6 and I1, eachincluding a stopping condenser I9, connects the output circuit of theamplifier to the amplifier input circuit, to produce negative feedbackof normal transmission, for example, for reducing distortion. Thefeedback loop for this negative feedback may be referred to as the mainfeedback loop; and the feedback around this loop of normal transmissionmay be called the normal or main feedback;

The gain reduction effected by this feedback may be large as, forexample, several times ten decibels. The gain of the amplifier withoutfeedback should then correspondingly exceed the gain with feedback. Forinstance, the gain without feedback may be of the order of decibels, andthe feedback may reduce the gain to, say, a value of the order of 35decibels.

With the two-stage balanced amplifier, the normal feedback is obtainedin the desired phase by 65 reversing the feedback leads I6 and I1, i.e'., crossing them over from one side of the push-pull circuit to theother side, as disclosed in E. Peterson Patent 1,955,827, April 24,1934. Thus, lead I6 is connected from the plate of tube 4 (which is 60in the upper side of the push-pull circuit) to the bridge circuit II inthe input circuit of tube 3 (which is'in the lower side of the push-pullcircult); and lead I! is similarly connected between the plate -of tube4' (which is in the lower side of 5 the balanced circuit) to the inputbridge circuit II of tube 3 (which is in the upper 'side of the balancedcircuit). The bridge circuits II and II. render the feedback pathcomprising connections l6 and I1 conjugate to windings I2 and I2 andcircuit I at balance of the bridges, as pointed out in theabove-mentioned Peterson patent.

Due to unbalance of the push-pull circuit, there may be undesiredpropagation of transmission through the push-pull stages, thistransmission 76 being propagated in the parallel mode, i. e., with ltube 3';

the two tubes of the push-pull stage effectively in parallel in the caseof each stage. This transmission may be called the paralleltransmission;

Since this parallel transmission maybe fed back through the feedbackpath comprising the feedback connections I6 and I1; tendency towardsinging around the main feedback loop may result. Such singing will bereferred to as parallel singing or over-all parallel singing.Unfortunately, the parallel singing tendency may be strong, since theamplifier in the main feedback loop has an even number of stages and mayinclude tubes suchas 3 and 3', with large amplification constants. Meansfor reducing the parallel singing tendency will be describedhereinafter.

-A plate current supply source 25 and a grid bias potential source I22are shown for tubes 4 and 4. The source I22 may bias the tubes forso-called class B operation in' which the space current of the tube isinterrupted for the order of half the period of the waves to beamplified. Further, the source I22 may maintain the grids alwaysnegative during operation of the amplifier. The grid biasing potentialsfor the tubes 4 and 4' are supplied through-high resistances 28 and 29,respectively.

The plate of tube '3 is connected to the grid of tube 4 through astopping condenser 21; and the plate of tube 3 is connected to the gridof tube 4' througha stopping condenser 21'.

A plate current supply source 40 supplies direct space current. for .thetubes 3 and 3 through choke coils 3I and"32, respectively, which are'closely coupled. The source 40 comprises two sources M and 42 seriallyconnected by an impedance 43 shown, by way of example, as an inductanceelement 44 and a capacity 45 in parallel.

- (Sources 42 and 25 may be common, if desired).

of parallel transmission around the main feedback loop), withoutmaterially reducing the amplifier gain (1. e.,' without reducing thegain of the amplifier between circuits I and 2 for normal transmission).Since the impedance 43 is in the grid mid-branch of the push-pull stagecomprising tubes 3 and 3', the normal transmission in the grid circuitsof these tubes is in one direction or phase in impedance 43 at any giveninstant for transmissiom through tube 3 and is in the opposite directionor phase in impedance 43 at the sameinstant for transmission through andso the normal transmission produces. no voltage drop across impedance43' in these grid circuits. However since it is of course impractical tomake the parallel opposing impedance of coils 3| and 32 zero, paralleltransmission through the two sides of the push-pull stage may becomeappreciable. For such'parallel transmission, in impedance 43 the platecurrent of tube 3 passing through impedance 43 and coil 3| at any giveninstant has the same direction or phase as the plate current of tube 3passing through impedance 43 and coil 32; so the of the small,

condition the tubes.

to prevent a series The close coupling of coils 3| and" 32 and likewisethe close coupling of coils 8 and 8', tends to short-circuit theparallel singing path and elim--v inate parallel singing. The mid-branchfeedback around the driver stage is effective in preventing parallelsinging regardless of this close coupling, and is especially useful forthis purpose when the close coupling is inadequate for the purpose,

By having the resistance of the inductance element 44 small, the directcurrentvoltage drop across the impedance 43 can be made small. Moreover,by having the voltage of source 4| equal and opposite to this directcurrent voltage drop across 43, the direct current potential of ,thecathodes of tubes 3 and 3 can'be kept at ground potential or thepotential .of the terminal of impedance 43 electrically remotefrom thecathodes, negative grid bias for the tubes being supplied by a voltagesource 46. The source 46 may maintain the control grids of the tubesalways negative during operation of the tubes and condenser serving inconnection with the inductance 44 as the impedance across which theauxlliaryfeedback, voltage is produced, may have a capacity of the orderof 800 micro-rriicrofarads, for example.

Feedback resistors 50 are shown in leads l6 and I! for con-trolling theamount of feedback. Small capacities 5| are shown in shunt to theseresistors. These capacities reduce the phase shift around the mainfeedback loop at very high frequencies sing,.i. e.) singing duetotransmission propagated through the push-pull stages in the, push-pullmode. The value of these capacities is not critical. Each may have avalue of the order of 50 micro-microfarads, for example.

Fig. 2 shows an amplifier'circuit similar to that of Fig. 1, but with asingle tube'orsingle-sided driver stage connected to circuit I by aninput transformer H0 and driving the push-pull stage through a couplingcircuit comprising a plate circuit resistor l3l for tube 3, the couplingcon- 21 and a balanced retard coil. 'or' autodenser transformer havingitsbalanced windings I28 and I29 tightly coupled. Over-all negativefeedback is obtained by a feedback path through connection II whichgives the desired revePse phase'of feedback voltage by connecting theplate of tube 4' to the input bridge H. v Using a singletube in thefirst stage not only reduces the required amount of apparatus, butreduces the powerdraln of the complete ampli-'- ,fier. The latter factoris especially important under no-load conditions when the power drainhigh-gain driver stage is comparable to the drain of the class B powerstage. Such a two-stage, three-tube negative feedback amplifier issubject to a kind of over-all singing which may be called parallelsinging and which is somewhat similar. to the over-all parallel singingdiscussed in connection with the four-tube circuit of Fig. l. The modeof oscillation-is such that the output tubes operate inparallel, (thatis, with the potentials of the two grids the same ine -steadpf 180degrees out of phase). though the oscillations are transmitted throughthe feedback of the interstage 'autotransformer, while thh out-' for-class A operation. The

' normal transmission;

3 put tubes faces the parallel opposing. impedance of the outputtransformer. transformers had their balancedcoils perfectly coupled andhad zerp resistance, such mode of oscillation (i. e., this over-allsingingwith parallelfoperation of, the two sides of the' power stage).would be impossible; but such construction is, of course, impracticable.

To reduce the over-all parallel singing tendency, an impedanceshown,'for example, as an inductance I43, is "connected in themid-branch of the balanced input circuit and the mid-branch of thebalanced output circuit of the. push-.pull

If either of these stage. This inductance functions similarly to theinductance 43- of Fig. 1. It produces local negative feedback ofparallel transmission around the stage in which'it' isconnected,lowering the gain of that stage for parallel transmission and serving toreduce the over-all parallelsinging tendency.

' In tests of a voice frequency, negative feedback, two-stage,three-tube amplifier, such as the amplifier of Fig. 2, two types ofparallel singing were feedback was used. Although the frequency of thefirst type of sing- (20 to kilocycles) was not encountered. Neither typeoccurred except when.

greatlydiscriminated against by the output transformer, due to thebalance in the-parallel path-no appreciable output was obtained in theload circuit at the singing frequency. This singing was stopped byusingthe' impedance I43 in the commomplate-grid mid-branch of the classB stage, the specific impedance employed being an a inductance of 22milhenrys. v

The second type of singing occurred ,at critical values of load(transmission level) A Braun tube pattern showed the singing as highlydamped oscillations occurring over a fraction of the fundamentaicycle'a'nd synchronized in fre- J quency with a high harmonic of thefundamental This singing was stopped by frequency applied.

1 and shunt capacity, 62

inserting a resistance in the balanced stage grid mid-branchofsuchvalues as to approximately resonate out the parallel opposingreactance of the .interstage coupling retard I28, I29 near the singingfrequency.

To avoid increase of third ordei', distortion and consequent limitationof the fundamental output available, it was found desirable to keep theimpedance l43- introduced in the plate circuit of the class B stage 'assmall as possible. h

The circuit of Fig. 3 avoids the necessityof introducing such impedancein the plate circuit of,

the class B'stage and has the advantage of in-,

'troducing the local feedback in the linear, highgain driver stage; yetit retains the'feature that the auxiliary or local negative feedback isso effected as to avoid reducing thegain for normal transmission or, inother words, to avoid mate- .rlally reducing the amplifier gain.

. This circuit is a two stage, three-tube, negative feedback amplifiergenerallyv similar to that of Fig. 2. However, the feedback voltage forthe auxiliary negative feedback is derived from a feedback impedance ina portlon'of the grid circuit of the class B stage that is traversed bythe parallel transmission to the exclusion of Ashe and the voltage soderived is fed back to the grid of the tube in the high-gain lineardriver stage.

This feedback impedance is shown as a resistance 248 in the mid branchof the balanced grid circuit of the class B stage. The cathodes of bothstages are shown grounded, and the ter--.

minal of the resistance :43 electrically remote from ground isshown'connected to the input bridge I I through a conductor 244 andaportion of the grid-biasing battery I22 sufficient to supply the gridbias for tube 3. Y

' Any voltage drop that appears across the impedance 243 is fed backinto the grid circuit of the driver stage. Any voltage applied to thetwo grids of the balanced stage in parallel, i. e., in the singing mode,will appear across this impedance 243 and, therefore, will be fed back,reducing the gain of the driver stage for that mode of transmission andreducing over-all parallel singing tendency. However, any voltage whichappears across the two grids of the balanced stage in series willbalance out across the impedance 243 and hence this impedance will notaffect the gain of the amplifier for normal transmission, or, in otherwords, will not materially reduce the over-all amplifier gain.

What is claimed is:

1. An amplifier with a feedback loop comprising a push-pull stage havinga mid-branch circuit and connections that introduce a phase re verse] intransmission around the loop when the transmission is in the push-pullmode rough the push-pull stage but not when the tra smission is in theparallel mode through the pushpull stage, and means comprising afeedback impedance in said mid-branch circuit producing negativefeedback around a portion of the amplifier stages in said'loop fortransmission whose passage through said push-pull stage is in theparallel mode but not for transmission whose passage through saidpush-pull stage is in the push-pull mode.

2. An amplifier with a feedback loop comprising a push-pull stage havinga mid-branch circuit and connections that introduce a phase reversal intransmission around the loop for transmission in the push-pull modethrough the pushpull stage but not for transmission in the parallel modethrough the push-pull stage, and means for reducing parallel singingtendency in the amplifier without materially lowering the amplifiergain, said means comprising a feedback impedance in said mid-branchcircuit producing feedback around a portion of the amplifier stages insaid loop that reduces the gain around said loop for transmission takingplace in the parallel mode through said push-pull stage, said portion ofthe amplifier stages including a stage having but one amplifyingelement. g

3. An amplifier with a feedbackloop producing negative feedback aroundan even number of stages including a push-pull stage, and meansproducing negative feedback around an odd number of those stages in. amode that suppresses parallel singing in the amplifier withoutmaterially reducing the amplifier gain.

4. An amplifier with a feedback loop producing negative feedback aroundan even number of mode.

- a,1os,4ee v stages including a push-pull stage and another stagehaving but one amplifying element; and means producing around an oddnumber of those stages including said other stage, feedback that reducesthe gain around said loop for transmission in the parallel mode throughsaid push-pull stage without reducing the gain around said loop fortransmission in the push-pull mode through said push-pull stage.

5. An amplifier with a normal negative feedback channel around afeedback loop including a single-sided stage feeding a push-pull stage,and a circuit feeding back from the common branch of the input circuitof the push-pull stage to the single-sided stage waves that, withoutreducing the gain around the normal feedback channel, reduce tendencytoward singing around a feedback channel including the two sides of thepush-pull stage in parallel.

.6. An amplifier with a feedback loop that in-- I cludes a push-pullstage and produces negative feedback in said amplifier for transmissionin the push-pull mode through said stage, and means producing feedbackfrom a mid-branch circuit of said push-pull stage around an odd numberof amplifier stages in said loop, said stages in-- cluding saidpush-pull stage.

7. An amplifier comprising a push-pull stage having a mid-branchcircuit, a normal negative feedback channel around a feedback loopincluding said push-pull stage, and connections comprising a feedbackimpedance in said mid-branch circuit feeding back in the amplifier wavesthat, without reducing the gain around the normal feedback channel,reduce tendency toward singing around a feedback channel including thetwo sides of the push-pull stage in parallel.

8. An amplifier with a feedback loop that includes a push-pull stage andproduces negative feedback in said amplifier for transmission in thepush-pull mode through said stage, and means producing feedback from amid-branch circuit of said push-pull stage around a feedback loop havingan odd number of amplifier stages, one of which is in said loop.

9. The method of operating a signal wave a mplifying system with apush-pull stage which comprises producing negative feedback in thesystem by transmission of the signal waves in a given mode around afeedback loop that includes said stage, producing positive feedbackaround said loop of waves transmitted through said Push-pull stage inthe parallel mode of transmission, and reducing the gain around the loopfor the latter waves by auxiliary negative feedback in a portion of theloop of those waves to the exclusion of waves transmitted'in said givenJOHN G. KREER, J ii.

